Thrombosis Pathology — Virchow Triad

Key Mechanism 1: Endothelial Injury (Vessel Wall)

• Prostacyclin (PGI₂) and nitric oxide (NO) production (antiplatelet and anticoagulant effects)
• Expression of thrombomodulin and protein C receptor (anticoagulant pathways)
• Surface heparan sulfate (anticoagulant)
• CD39/CD73 expression (adenosine diphosphate hydrolysis)

Key Mechanism 2: Blood Flow Abnormalities (Stasis and Turbulence)

• Platelet margination and increased wall contact
• Endothelial shear stress and activation
• Prolonged residence time of clotting factors in contact with thrombogenic surfaces

• Atrial fibrillation (blood pooling in dilated atrial appendage)
• Ventricular aneurysm (post-myocardial infarction)
• Prosthetic heart valves (abnormal hemodynamics)
• Mitral stenosis (narrowed orifice)
• Immobilization (venous stasis in lower extremities)

Key Mechanism 3: Blood Hypercoagulability

Inherited hypercoagulable disorders

• Factor V Leiden (most common; resistance to activated protein C due to Arg506Gln mutation affecting Factor Va cleavage)
• Prothrombin gene mutation (G20210A; elevated Factor II levels)
• Protein C deficiency (impaired inactivation of Factors Va and VIIIa)
• Protein S deficiency (cofactor for protein C)
• Antithrombin deficiency (reduced Factor II, VII, IX, X, and thrombin inhibition)
• Dysfibrinogenemia (abnormal fibrinogen function)

Acquired hypercoagulable disorders

• Malignancy (cancer cells express tissue factor, produce procoagulants; chemotherapy increases thrombosis risk)
• Pregnancy and postpartum period (increased Factors V, VII, VIII, X, XII; decreased protein S; increased vWF)
• Oral contraceptives and hormone replacement therapy (estrogen-mediated)
• Antiphospholipid syndrome (APS) (IgG/IgM antibodies against phospholipid-binding proteins; lupus anticoagulant, anticardiolipin antibodies)
• Nephrotic syndrome (urinary loss of anticoagulants; increased hepatic synthesis of procoagulants)
• Disseminated intravascular coagulation (DIC) (pathologic activation of entire coagulation cascade)
• Smoking (increases vWF, decreases protein C; endothelial dysfunction)
• Sepsis (lipopolysaccharide activates TF pathway)

Major Cause 1: Atherosclerotic Plaque Rupture and Vessel Wall Injury

• Coronary artery thrombosis (myocardial infarction)
• Cerebral artery thrombosis (ischemic stroke)
• Peripheral arterial thrombosis
• Surgical trauma, cardiac catheterization
• Indwelling vascular catheters
• Pacemaker leads
• Cardiac valve replacement
• Traumatic vascular injury

Major Cause 2: Cardiac Abnormalities (Low-Flow States)

• Atrial fibrillation (most common arrhythmia predisposing to thrombosis)
• Dilated cardiomyopathy (reduced ejection fraction; endocardial damage)
• Acute myocardial infarction (wall motion abnormality; ventricular aneurysm; endocardial scarring)
• Valvular disease (mitral stenosis; prosthetic valves)
• Cardiac tumors (myxoma; endocardial contact)
• Dilated atrial appendage
• Restrictive pericarditis (impaired ventricular filling)

Major Cause 3: Venous Stasis and Immobility

• Prolonged bed rest or immobilization (hip fracture, surgery, post-operative period)
• Long air/car travel (economy class syndrome)
• Obesity (immobility; increased intra-abdominal pressure)
• Pregnancy and postpartum period (compression of iliac veins by gravid uterus)
• Malignancy (direct vessel obstruction; hypercoagulability)

Major Cause 4: Hypercoagulable States (as detailed above)

Major Cause 5: Infection and Inflammation

• Sepsis (endotoxin-mediated TF activation; DIC)
• Vasculitis (endothelial injury)
• Bacterial endocarditis (vegetations; microbial procoagulants)
• Inflammatory bowel disease (hypercoagulability)

Cardinal Symptoms and Signs (Depend on Location and Vessel Affected)

Arterial Thrombosis

• Acute limb ischemia (sudden onset of pain, pallor, pulselessness, paresthesias, paralysis — "6 Ps")
• Acute myocardial infarction (chest pain, dyspnea, diaphoresis; ST-elevation on ECG)
• Acute ischemic stroke (sudden neurological deficit; focal weakness, speech disturbance, visual loss)
• Acute mesenteric ischemia (severe abdominal pain disproportionate to exam; bloody diarrhea if late)
• Splenic or renal infarction (flank pain; hematuria)

Venous Thrombosis

• Deep vein thrombosis (DVT) of lower extremity:
• Unilateral leg swelling (edema), pain, erythema, warmth
• Positive Homan sign (pain on dorsiflexion; low sensitivity/specificity)
• Positive Wells score
• Dilated superficial veins

• Pulmonary embolism (PE) (if proximal DVT propagates):
• Sudden dyspnea, chest pain (pleuritic), syncope
• Tachycardia, hypoxemia, elevated D-dimer
• Wedge-shaped infarct on imaging (if small peripheral vessel involved)

• Upper extremity DVT (arm swelling; may follow central line placement)
• Cerebral venous sinus thrombosis (CVST) (headache; seizures; focal neurological signs)
• Portal vein thrombosis (abdominal pain; ascites; variceal bleeding)
• Mesenteric venous thrombosis (abdominal pain; hyperamylasemia if pancreatic duct involved)

Physical Exam Findings

• Limb edema, erythema, warmth (venous)
• Cool, pale, cyanotic extremity (arterial)
• Diminished or absent pulses (arterial)
• Calf tenderness (DVT)
• Unilateral asymmetry in limb circumference
• Phlegmasia cerulea dolens (massive DVT causing venous gangrene; blue, swollen, painful limb)

Lab and Imaging Correlates

• Elevated D-dimer (fibrin degradation product; sensitive but not specific for thrombosis)
• Prolonged PT/INR (if on warfarin)
• Elevated prothrombin time, activated partial thromboplastin time (if hypercoagulable)
• Arterial blood gas (hypoxemia in PE)
• Troponin elevation (myocardial infarction)
• Elevated lactate (mesenteric ischemia; tissue hypoxia)
• CT pulmonary angiography (CTPA) (PE diagnosis)
• Compression ultrasound or venography (DVT diagnosis)
• ECG changes (ST elevation in STEMI; T wave inversion in unstable angina)
• Angiography (gold standard for arterial imaging)

Histological Findings

Early thrombus (< 12 hours)

• Platelet-rich white thrombus at site of vessel wall injury
• Predominance of platelets and fibrin with minimal red blood cells
• Layering pattern reflecting laminar flow conditions

Laminar or lines of Zahn

• Alternating bands of platelets/fibrin (white) and red blood cells (red)
• Characteristic of antemortem thrombus (formed while blood flowing)
• Indicates formation occurred during life with continued blood flow around clot

Postmortem clot (autem clot)

• Homogeneous dark red appearance ("currant jelly" clot)
• No lines of Zahn
• No platelet layering
• Histology: Even distribution of RBCs throughout fibrin with minimal platelets
• NOT a true thrombus (no pathologic adhesion to vessel wall; no inflammatory response)

Organizing thrombus (> 12 hours to days)

• Leukocytic infiltration (neutrophils initially, then macrophages)
• Fibrinolysis and clot dissolution
• Ingrowth of granulation tissue from vessel wall margins
• Neovascularization (capillary loops)
• Progressive organization into fibrous tissue

Mature organized thrombus (weeks to months)

• Fibroblasts and collagen replacing thrombus
• Smooth muscle cell proliferation
• Recanalization (formation of new vascular channels within thrombus, permitting limited blood flow)
• Endothelialization of new channels

Gross Pathology Appearance

• White (pale) thrombus: Arterial; pale, granular appearance; firmly adherent to vessel wall; lines of Zahn visible
• Red (dark) thrombus: Venous; dark red, gelatinous; less firmly adherent; postmortem clot histologically similar but lacks true adhesion
• Mixed thrombus: Combination of white and red components (common in cardiac chambers)
• Mural thrombus: Adherent to vessel wall, not completely occluding lumen (e.g., post-MI ventricular wall)
• Occlusive thrombus: Completely fills vessel lumen

Lab Values and Coagulation Studies

• Prothrombin time (PT): Evaluates extrinsic and common pathways (Factors II, V, VII, X, fibrinogen)
• Activated partial thromboplastin time (aPTT): Intrinsic and common pathways (Factors II, V, VIII, IX, X, XI, XII, fibrinogen, prekallikrein, high-molecular-weight kininogen)
• Thrombin time: Direct fibrinogen-to-fibrin conversion
• Platelet count: Baseline assessment
• D-dimer: Elevated in thrombotic states (sensitive but nonspecific)
• Fibrinogen level: Depleted in DIC
• Specific factor assays: If inherited hypercoagulability suspected
• Protein C, Protein S activity: Decreased in inherited deficiency or DIC
• Antithrombin III activity: Decreased in inherited deficiency or DIC
• Anticardiolipin antibodies, lupus anticoagulant: Antiphospholipid syndrome screening
• Factor V Leiden and Prothrombin G20210A genetic testing

Diagnostic Criteria

DVT (Wells score for pre-test probability)

• High clinical suspicion confirmed by compression ultrasound (loss of vein compressibility; hyperechogenicity)
• D-dimer elevation (supports but does not diagnose)
• Venography: Gold standard (rarely needed if ultrasound conclusive)

PE

• CT pulmonary angiography (CTPA): Gold standard; shows filling defect
• Ventilation-perfusion (V/Q) scan: Alternative (perfusion defect without ventilation defect)
• D-dimer: High sensitivity (low specificity); negative D-dimer rules out PE in low-risk patients
• Wells score and PERC rule: Clinical prediction tools

Arterial thrombosis

• Angiography: Gold standard; shows abrupt vessel cutoff
• CT or MR angiography: Excellent visualization
• Duplex ultrasound: For peripheral vessels
• Acute coronary syndrome: ECG (ST elevation vs. non-ST elevation), troponin elevation, angiography

Hypercoagulability workup (if recurrent or unprovoked thrombosis)

• Factor V Leiden, prothrombin G20210A mutations
• Protein C, protein S, antithrombin activity
• Anticardiolipin antibodies, lupus anticoagulant (APS screen)
• CBC (platelet count), PT/INR, aPTT
• Consider thrombophilia panel after acute event resolves (some anticoagulants lower natural anticoagulant levels)

First-Line Treatment with Rationale

Unfractionated heparin (UFH) or Low-Molecular-Weight Heparin (LMWH)

• Mechanism: Binds and potentiates antithrombin III, which inhibits Factors IIa and Xa
• Onset: Immediate (minutes for UFH IV); anticoagulation within hours
• Indication: Acute thrombosis requiring rapid anticoagulation
• Advantages: Reversible (protamine sulfate neutralizes UFH); short half-life; can transition to warfarin or other agents
• LMWH: Predictable pharmacokinetics; subcutaneous dosing; preferred in many settings
• Monitoring: Activated partial thromboplastin time (UFH); anti-Xa activity (optional for LMWH)

Warfarin (Vitamin K Antagonist)

• Mechanism: Inhibits vitamin K-dependent factors (II, VII, IX, X) and natural anticoagulants (Protein C, S)
• Onset: Slow (2-7 days); initial paradoxical hypercoagulability if not bridged with heparin
• Indication: Long-term anticoagulation (maintenance therapy)
• Monitoring: INR target typically 2-3 (therapeutic range varies by indication)
• Advantage: Oral; well-established; reversible (vitamin K, fresh frozen plasma, prothrombin complex concentrate)
• Disadvantage: Narrow therapeutic window; many

Complications of the thrombus itself

• Embolization: fragments detach and lodge downstream. Proximal (iliofemoral/popliteal) DVT embolizing to the pulmonary arteries is the feared event; massive PE with obstructive shock is an emergency — hypotension, distended neck veins, RV dilation with septal flattening on bedside echo, and troponin/BNP elevation from RV strain.
• Paradoxical embolism: venous thrombus crossing a patent foramen ovale or ASD to the systemic circulation, producing stroke or limb ischemia in a patient with a DVT — the classic "venous clot, arterial event" stem.
• Infarction: complete occlusion produces coagulative necrosis (red/hemorrhagic in dual-supply or loose tissue such as lung and bowel; pale in kidney, spleen, heart).
• Post-thrombotic syndrome: recanalization destroys venous valves → ambulatory venous hypertension with chronic edema, hemosiderin hyperpigmentation, and medial malleolar ulceration.
• Chronic thromboembolic pulmonary hypertension (CTEPH): organized, fibrotic thrombus is not lysed → fixed pulmonary vascular obstruction. Suspect with dyspnea persisting months after PE; per AHA/ACCP guidance the V/Q scan, not CTPA, is the screening study because it is more sensitive for chronic organized clot.
• Phlegmasia cerulea dolens: near-total venous outflow obstruction → arterial compromise, venous gangrene, compartment syndrome. Vascular emergency requiring urgent thrombolysis/thrombectomy.

Complications of therapy

• Major hemorrhage: intracranial bleeding is the emergency. Reversal per ACC/CHEST guidance — protamine for unfractionated heparin (partial for LMWH), vitamin K plus 4-factor PCC for warfarin, idarucizumab for dabigatran, andexanet alfa for oral factor Xa inhibitors.
• Heparin-induced thrombocytopenia (type II): IgG against PF4–heparin complexes cross-links platelet FcγRIIa → platelet activation. Platelets fall roughly by half around days 5–10 and the patient clots rather than bleeds. Per ASH guidance, stop all heparin (including flushes/coated lines), score with the 4Ts, and start a non-heparin agent (argatroban, bivalirudin, or fondaparinux); withhold warfarin until platelets recover to avoid venous limb gangrene, and avoid prophylactic platelet transfusion.
• Warfarin-induced skin necrosis: protein C has the shortest half-life of the vitamin K–dependent factors, so early warfarin without heparin bridging is transiently prothrombotic → dermal microvascular thrombosis in fat-rich skin, especially with underlying protein C deficiency.
• Long-term heparin: osteopenia and aldosterone suppression with hyperkalemia.
• IVC filter: filter thrombosis, migration, and increased recurrent DVT — retrieve when the bleeding contraindication resolves.

• Lines of Zahn are the single most tested morphologic fact: alternating pale platelet/fibrin layers and red RBC-rich layers prove the thrombus formed antemortem in flowing blood. A homogeneous dark "currant jelly" clot with a yellow "chicken fat" supernatant and no wall attachment is a postmortem clot, not a thrombus.
• Arterial vs venous thrombus determines the drug class: arterial (white, platelet-rich, high-shear, plaque rupture) responds to antiplatelet therapy; venous (red, fibrin/RBC-rich, stasis) responds to anticoagulation. The classic distractor is treating a coronary or cerebral event as a pure anticoagulation problem, or a DVT with aspirin alone.
• Factor V Leiden is the most common inherited thrombophilia in patients of European descent — mutant Factor Va resists cleavage by activated protein C. Screen with an activated protein C resistance assay, confirm by DNA testing. Do not order a thrombophilia panel during the acute event or while on anticoagulation, since protein C/S and antithrombin levels are consumed or drug-suppressed.
• Antiphospholipid syndrome is the paradox: a prolonged aPTT that does not correct on mixing study, plus a false-positive VDRL/RPR, in a patient who thromboses. Per ACR/EULAR and ASH guidance, warfarin — not a DOAC — is preferred, particularly in triple-positive or arterial-event patients.
• Single best next step in suspected PE: if hemodynamically unstable, do not send the patient to CT — bedside echo plus empiric anticoagulation and consideration of systemic thrombolysis. If stable, risk-stratify (Wells/PERC), then D-dimer for low–intermediate probability, CTPA for high probability. A negative D-dimer only helps when pretest probability is low.
• Migratory superficial thrombophlebitis (Trousseau syndrome) in an older patient points to occult visceral adenocarcinoma, classically pancreatic — mucin and tissue factor drive it.
• Homan sign is the trap: too insensitive and nonspecific to rule DVT in or out; compression ultrasound showing a noncompressible vein is the answer.
• Nonbacterial thrombotic (marantic) endocarditis — sterile fibrin–platelet vegetations on undamaged valves from malignancy or SLE (Libman–Sacks) — is the hypercoagulability-driven cause of embolic stroke that examiners hide behind a "culture-negative endocarditis" stem.